The present invention relates an apparatus for forming stable and internally regular bell mouth shapes on the terminal junction section of pipes made of thermoplastic material, of the type operating on a terminal junction section, previously heated to the plastic state and comprising:
a first portion of the type with radially movable mechanical sectors, whose length, measured along the longitudinal axis of the mandrel is equal to the length of the bell mouth and movable from one first extreme retracted position in which the first portion presents such a diameter as to be freely introduced into the terminal section of the pipe, to a second extreme expanded position which dilates the entire part of the terminal section that surrounds it, thereby forming a temporary bell mouth, of maximum dilation, whose temporary inner diameter, corresponding to the outer one of said first portion in position of extreme expansion, is greater than the final nominal inner diameter of the definitive bell mouth, said first portion being heated to a temperature that is greater than or equal to that of the terminal section, so that the latter does not release heat during its plastic dilation to form the temporary bell mouth of maximum dilation, the first portion then returning to the first extreme retracted position;
a second portion, geometrically fixed, presenting an outer diameter that is equal to the nominal and final inner diameter of the definitive bell mouth, said second portion being able to be introduced into the temporary bell mouth of maximum dilation, after the complete intervention of said first portion which, in the retracted position, allows the free subsequent introduction of said second portion into the temporary bell mouth of maximum dilation, said second portion being able to calibrate exactly the inner diameter of the bell mouth, subject to spontaneous retraction after the previous dilation and being cooled below the temperature of the terminal section, thereby rapidly bringing the definitive bell mouth shape to ambient temperature once it has reached the definitive nominal diameter.
In this technological sector there are difficulties to overcome, linked to the necessary precision that the bell mouth shapes must provide at least internally, in order to obtain an effective junction between multiple sections of pipe, as well as to the problem of the shrinking by the thermoplastic material, after the definitive formation of the bell mouth shape. It is well known that the molecules of the thermoplastic materials that are subjected to dimensional change, remember the previous physical state and tend to return to the original state. Hence when bell mouth shapes are effected, which increase the diameter of the pipe, extruded when hot, the pipe would tend to return to the previous state, i.e. with a smaller diameter than the one formed with the bell mouth shaping operation. Moreover, the aforesaid shrinking varies according to the thermoplastic material in use (polypropylene, PVC, polyethylene, polyolefins in general).
To solve the problem of the precision of the internal measurements of the bell mouth shaping, techniques have for instance been adopted whereby the bell mouths are shaped with mechanical mandrels conformed according to the bell mouth shape to be obtained. They were forcedly introduced into the terminal section of pipe to be given a bell mouth shape, previously heated to its plastic deformation temperature, provided the pipe to be treated had sufficient thicknesses so that it would not collapse during the introduction of the mandrel. In the case of bell mouth shapes that comprise internal annular gaskets, for better sealing, radially expandable and subsequently retractable mechanical sectors are provided to allow the formation, under heat, of the gasket seat in the bell mouth.
In regard to the aforesaid technique, reference is made to European Patent EP. N.052.581, of Oct. 14, 1981, in the name of the same Applicant, which provides a good description of the wedge shaped actuation means (with inclined planes for constraining each mechanical sector) and the conformation of the mechanical sectors themselves, so shaped as to constitute, when expanded, a homogeneous bearing surface for shaping, from within, the annular seat for the bell mouth gasket.
Techniques have also been studied that provide for exerting inward pressure from the exterior, on the pre-heated terminal section of the pipe, countered by an internal mandrel, but these techniques are not applicable to the case at hand, which aims at applying a precise bell mouth shape without material shrinking, also for corrugated pipes, which clearly cannot be treated under heat, with pressure by fluid or mechanical means from the exterior and which notoriously have rather low thicknesses, in favor of the corrugations and/or external fins or contours.
In order to solve the problem of the shrinking by the thermoplastic material, in 1983 the Applicant perfected an apparatus that provides a shaping chamber defined internally by a mandrel that presents the final shape and nominal measurements of the bell mouth to be obtained, and externally by moulds whereon the terminal section of pipe to be treated is preliminarily made to adhere, by means of pressurized air coming from the interior of the mandrel. The terminal section is thus expanded beyond its required nominal size, keeping it for a certain time in contact against the aforesaid external moulds, then letting the bell mouth so formed retract spontaneously and calibrate itself with precision on the underlying mandrel, whereon conduits for the outflow of the pressurized air are provided.
It has been observed that in such cases the shrinking by the thermoplastic material was absent or in any case highly reduced after the complete formation of the bell mouth shape and an adequate precision was obtained, since an internal calibration of the bell mouth shape was always present.
Unfortunately, this technique too is clearly not applicable to pipes made of thermoplastic material that are corrugated or externally provided with fins and/or contouring.
Therefore an apparatus has been realized by the same Applicant, as per European patent application no. 98839360.8, of Jun. 11, 1998 and as per the foreword to the description. In this patent documentation, the device able to produce the forced dilation is a mandrel with mechanical sectors, radially movable from a first retracted position, in which it can freely be introduced into the terminal section, to a second expanded position, in which it dilates the terminal section until forming the temporary bell mouth shape with greater than nominal diameter.
This apparatus, although it is valuable to realize bell mouths on corrugated or contoured pipes, can nonetheless be further improved. It has been noted that this technique causes, on the inner surface of the bell mouth, some impressions due to contact with the lateral extremities of the radially moving sectors, during their passage from the retracted position of minimum size to that of maximum expansion or maximum size.
Therefore the aim of the invention, as it is characterized in the enclosed claims, is to eliminate the aforementioned drawback, providing additional advantages and retaining those already present.
The invention retains the advantage of being applicable to any type of thermoplastic pipe available on the market, including corrugated ones and/or those with external contouring (for instance, of the kind called ultrarib), notoriously with reduced thickness, realizing stable bell mouths.
It further provides the advantage of allowing very precise bell mouth shapes, with perfectly regular inner surfaces and without any kind of impression, even when the dilation of the terminal section beyond the nominal size of the bell mouth to be obtained is effected with mandrels of the kind with radially movable mechanical sectors.
Not the least advantage is also to provide a considerable rapidity of execution, a phase having been provided whereby the calibration operation is accelerated, by means of air pressure from the exterior, which can also be used effectively for corrugated pipes, which as is well known have never been subjected to said external pressures, due to the danger that they may implode.